5-wire resistive touch screen pressure measurement circuit and method

ABSTRACT

A 5-wire touch screen system includes a touch screen ( 10 ) including a wiper ( 11 ) and a resistive layer ( 16 ) aligned with the wiper and first (UL), second (UR), third (LR), and fourth (LL) resistive layer contacts, wherein a touch on the screen presses a small portion of the wiper against the resistive layer, producing a touch resistance (R Z ) between them at a touch point on the resistive layer. The wiper and various contacts are selectively coupled to first (V DD ) and second (GND) reference voltages, respectively, to generate an analog touch voltage (V Z ) at the touch point. The wiper and various contacts are selectively coupled to an analog input ( 56 ) and a reference voltage input of an ADC ( 48 ) for converting the touch voltage (V Z ) to a digital representation. Analog voltages (V X ) and (V Y ) at the touch point are converted to corresponding digital representations by the ADC.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/657,046, filed on Jan. 13, 2010 (U.S. Pat. No. ______) which isincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to 5-wire touch screens, andmore particularly to systems and methods for accurately determiningtouch pressure/force applied on 5-wire touch screens.

FIG. 1 shows an exploded isometric diagram of a conventional 5-wireresistive touch screen 10 including a transparent bottom layer 14,coated with resistive film 16 and four conductive corner pads 15-1,15-2, 15-3 and 15-4 that can be connected to an outside contactterminal, and a top layer 12. (The layers need to be transparent toallow display or LCD (liquid crystal display) backlighting to passthrough.) FIG. 2 shows a section view of an implementation of theassembled version of the exploded view of touch screen 10 in FIG. 1,wherein top layer 12 typically is formed of polyester or polyethyleneterephthalate (PET) and is coated underneath with highly conductive(e.g., metal) transparent material to form wiper layer 11 (also referredto simply as “wiper 11”).

Transparent bottom layer 14 also is formed of PET, coated withtransparent resistive film 16, which usually is ITO (indium tin oxide).

Elastic and insulative spacers 22 separate top layer 12 from bottomlayer 14 so as to maintain a thin air gap 23 between them. Spacers 22are typically very thin, and are used to avoid a large difference in thetouch point contact resistance, which is dependent on where the touch islocated relative to the locations of the spacers, and also to avoidsubstantial variation in the “feel” for various locations of the touchpoint relative to the spacers.

Applying a touch pressure to the outer surface of top layer 12 pushes asmall touch contact area of wiper 11 against resistive ITO layer 16.When no touch pressure is present on top layer 12, it is separated fromthe bottom resistive layer 14 by spacers 22 and air gap 23.

The pressure of a touch on the upper surface of touch screen 10typically is detected by a conventional 5-wire touch screen controllerthat controls various drive signals applied to the passive resistance ofresistive layer 16 so as to facilitate measurement of various voltagesresulting from touching various locations on the top surface of touchscreen 10.

FIG. 3 shows an equivalent circuit of the idealized 5-wire resistivetouch screen 10 depicted in FIGS. 1 and 2. Transparent resistive layer16 of FIG. 2 is represented in FIG. 3 as a rectangular grid ofequivalent resistors having conductive terminals UL, UR, LL, and LR onits upper left, upper right, lower left, and lower right cornerscorresponding to conductive pads 15-1, 15-2, 15-4, and 15-3,respectively, in FIG. 1. Wiper layer 11 thus is directly over resistivelayer 16 and is connected to wiper contact terminal 35. Conductors orcorner terminals 15-1, 15-2, 15-4, 15-3, and wiper contact terminal 35are the 5 accessible conductors or “wires” of 5-wire touch screen 10. Ifcorner terminals UL and LL are connected by a conductor 27 and terminalsUR and LR are connected by a conductor 29 as indicated in the lowerportion of FIG. 3, then resistive layer 16 appears as a resistorconnected between conductors 27 and 29, as shown in the simplifiedequivalent circuit representation in the lower portion of FIG. 3.Similarly, if terminals UL and UR are connected together by conductor 26and terminals LL and LR are connected together by conductor 28,resistive layer 16 appear as a resistor connected between conductors 26and 28. A touch point area 31 on top conductive wiper 11 conducts touchpressure to a point or area 30 on the resistive grid when a touch isapplied on touch screen 10.

FIG. 4 shows an equivalent circuit similar to the equivalent circuitshown in FIG. 3 but further including the “touch resistance” 33 having avalue R_(Z) of a touch between contact area 31 on wiper 11 and contactarea 30 on ITO resistive layer 16. Touch contact areas 30 and 31 aresmall contact areas that occur as a result of touch pressure applied ontop layer 12 that presses small area 31 of wiper layer 11 against smallarea 30 of resistive layer 16. Note that wiper layer 11 is assumed to beof zero resistance in the equivalent circuit of FIG. 4.

Unfortunately, it is not presently practical to provide a highlyconductive (e.g., metal) contact wiper layer 11 that is sufficientlytransparent for the LCD backlighting applications in which touch screensoften are utilized. The wiper layer coat 11 on the lower surface of toplayer 12 is presently composed of nearly-transparent ITO resistivematerial, the same as resistive layer 16 on the upper surface of bottomlayer 14. As the result, the equivalent circuit of a practical 5-wiretouch screen 10 may be as shown in FIG. 5, where resistance 34 having avalue R_(wiper) represents the resistance of ITO resistive wiper layer11 between the touch area 31 and wiper contact terminal 35.

Typically, each of the two ITO resistive layers 11 and 16 isapproximately 90% transparent. Therefore, the top and bottom layers 12and 14 together are 90%×90%=81% transparent, theoretically. This is veryimportant, because lower transparency of the touch screen causes morepower to be dissipated in the LCD backlighting circuitry in order toprovide sufficient light intensity.

FIG. 6A is an equivalent circuit that is useful in explaining theprocess of determining the y-coordinate of a touch on a conventional5-wire resistive touch screen. Measurement of the y-coordinate includesapplying a voltage V_(DD) of voltage source 38 between conductor 26,which is connected to terminals UL (15-1) and UR (15-2), and conductor28, which is connected to terminals LL (15-4) and LR (15-3). Sensing they-coordinate location of the electrical contact at the touch point (notshown) is accomplished through conductive terminal 35 of wiper 11.Similarly, FIG. 6B is an equivalent circuit useful in explaining theprocess of determining the x-coordinate of a touch on the touch screen.Measurement of the x-coordinate includes applying a voltage V_(DD)between conductor 29, which is connected to terminals LR and UR, andconductor 27, which is connected to terminals UL and LL. Sensing thelocation of the electrical contact at the touch point is accomplishedthrough conductive point 35 of wiper 11.

More specifically, the above-mentioned touch screen controller to whichtouch screen 10 is coupled first applies the screen driving voltageV_(DD) of voltage source 38 between conductors 26 and 28, causingcurrent to flow uniformly across the screen from top to bottom in FIG.6A. The y-coordinate voltage V_(Y) is read from contact terminal 35 ofwiper 11, and is given by the expression

$\begin{matrix}{{V_{Y} = {\frac{V_{DD}}{R_{Y}} \times R_{Y\; 2}}},} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where the y-direction resistance R_(Y) between conductors 26 and 28 is aknown value that can be easily measured. R_(Y2) is the resistancebetween the touch point 30 and the negative (−) terminal of voltagesource 38. (R_(Y) and R_(Y2) are illustrated in FIG. 7A.)

Similarly, the touch screen controller applies the screen drivingvoltage V_(DD) of voltage source 38 between conductors 29 and 27 in FIG.6B, causing current to flow uniformly across the screen from right toleft. The x-coordinate voltage V_(X) is read from contact terminal 35 ofwiper 11, and is given by the expression

$\begin{matrix}{{V_{X} = {\frac{V_{DD}}{R_{X}} \times R_{X\; 2}}},} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

where the x-direction resistance R_(X) between conductors 27 and 29 is aknown value that can be easily measured. R_(X2) is the resistancebetween the touch point 30 and the negative (−) terminal of voltagesource 38. (R_(X) and R_(Y2) are illustrated in FIG. 7B.)

In addition to the foregoing touch screens, the closest prior art isbelieved to also include U.S. Pat. Nos. 6,246,394 and 7,215,330. U.S.Pat. No. 6,246,394 “Touch screen Measurement Circuit and Method”, issuedJun. 12, 2001 to Kalthoff et al., discloses a 4-wire touch screendigitizing system, and presents a method that measures the x and ycoordinates of a touch location. U.S. Pat. No. 7,215,330“Touch-Sensitive Surface Which Is Also Sensitive to Pressure Levels”,issued May 8, 2007 to Rantet, discloses a 4-wire touch screen thatincludes orthogonal conductive tracks 6 and 8 connected to resistivestrips along edges of the two screens that make up the touch-sensitivescreen, such that the x and y coordinates and the applied pressure canbe measured. The method measures the pressure or third or “z” coordinateof a touch point on a 4-wire resistive touch screen.

Touch screen users may occasionally bump a nearby article that impartsmechanical vibration to a touch screen that can cause the associatedtouch screen system to erroneously interpret touch location orerroneously interpret the vibration as an intentional touch. Also, usersmay inadvertently touch the screen surface. If the touch screen andassociated controller have the capability of measuring the touchresistance between the wiper layer and the resistive layer of the touchscreen, then a “sensitivity” threshold value can be established whichprevents erroneous touch interpretation due to mechanical vibration orlight extraneous touches on the touch screen surface. In someapplications, for example, interpreting Chinese characters being writtenon a touch screen or drawing of graphical features, varying amounts offorce/pressure applied to the touch screen surface by a stylus can beinterpreted as representing lines of varying width or darkness. Also,there are applications in which the above-mentioned sensitivitythreshold value can be utilized to prevent electrical noise, such as EMI(electro-magnetic interference), from causing touch interpretationerrors.

The prior 5-wire touch screen systems can only measure x- andy-coordinates, lack any method for obtaining third-coordinate orpressure data, and have limited capability for performing certainfunctions, such as signature verification, in which the pressure appliedto provide a valid signature can be very significant. Without thepressure measurement of the present invention for a 5-wire touch screensystem, the 5-wire touch screen system can generate only 2-dimensionalcoordinates, and therefore supports only 2-dimensional applications onthe touch screen surface.

Thus, there is an unmet need for a system that measures 3 touch pointcoordinate voltages developed in a touch screen panel to represent xcoordinates, y coordinates, and a touch point contact resistancecoordinate, respectively, between a wiper layer and a resistive layer ofa 5-wire touch screen.

There also is an unmet need for a system that measures 3 touch pointcoordinate voltages developed in a touch screen panel to represent xcoordinates, y coordinates, and a touch point contact resistancecoordinate between a wiper layer and a resistive layer of a 5-wire touchscreen, wherein the touch point contact resistance is utilized todetermine a touch point contact pressure or force.

There also is an unmet need for a touch screen system capable ofproviding improved signature verification by utilizing the touchpressure contact resistance in a 5-wire touch screen.

There also is an unmet need for a touch screen system capable ofproviding touch intensity measurements by utilizing the touch pointcontact resistance on a 5-wire touch screen.

There also is an unmet need for a touch screen system capable ofproviding touch sensitivity measurements by utilizing the touch pointcontact resistance on a 5-wire touch screen, wherein EMI(electro-magnetic interference) from the touch screen can bedistinguished from real touches or pressures.

There also is an unmet need for a touch screen system capable ofproviding touch sensitivity measurements by utilizing the touch pointcontact resistance in a 5-wire touch screen. wherein touch point sizeinformation can be determined.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system that measures 3touch point coordinate voltages developed in a 5-wire touch screen panelto represent x coordinates, y coordinates, and a touch point contactresistance coordinate, respectively, between a wiper layer and aresistive layer.

It is another object of the invention to provide a system that measures3 touch point coordinate voltages developed in a 5-wire touch screenpanel to represent x coordinates, y coordinates, and a touch pointcontact resistance, respectively, between a wiper layer and a resistivelayer, wherein the touch point contact resistance is utilized todetermine a touch point contact pressure or force.

It is another object of the invention to provide a touch screen systemcapable of providing improved signature verification by utilizing touchpoint contact resistance in a 5-wire touch screen.

It is another object of the invention to provide touch sensitivitymeasurements by utilizing touch point contact resistance in a 5-wiretouch screen.

It is another object of the invention to provide touch sensitivitymeasurements by utilizing touch point contact resistance in a 5-wiretouch screen, wherein EMI (electro-magnetic interference) from the touchscreen can be distinguished from real touches or pressures.

It is another object of the invention to provide touch sensitivitymeasurement by utilizing touch point contact resistance in a 5-wiretouch screen, wherein touch point size can be determined.

Briefly described, and in accordance with one embodiment, the presentinvention provides a 5-wire touch screen system that includes a touchscreen (10) including a wiper (11) and a resistive layer (16) alignedwith the wiper and first (UL), second (UR), third (LR), and fourth (LL)resistive layer contacts, wherein a touch on the screen presses a smallportion of the wiper against the resistive layer, producing a touchresistance (R_(Z)) between them at a touch point on the resistive layer.The wiper and various contacts are selectively coupled to first (V_(DD))and second (GND) reference voltages, respectively, to generate an analogtouch voltage (V_(Z)) at the touch point. The wiper and various contactsare selectively coupled to an analog input (56) and a reference voltageinput of an ADC (48) for converting the touch voltage (V_(Z)) to adigital representation. Analog voltages (V_(X)) and (V_(Y)) at the touchpoint are converted to corresponding digital representations by the ADC.

In one embodiment, the invention provides a 5-wire touch screen system(40) including a 5-wire touch screen sensor (10), a substantiallytransparent, substantially conductive wiper layer (11), a substantiallytransparent resistive layer (16) aligned with the wiper layer (11)wherein the resistive layer (16) includes first (UL), second (UR), third(LL), and fourth (LR) contact terminals, and a plurality of thin spacers(22) separating the wiper layer (11) and the resistive layer (16),wherein a touch on the wiper layer (11) presses a small portion (31) ofthe wiper layer (11) against the resistive layer (16) to form aresistive contact area (30) having a touch resistance (R_(Z)) betweenthe wiper layer (11) and the resistive layer (16), the touch resistance(R_(Z)) being inversely proportional to an intensity (P_(touch)) of thetouch. A controller (41) coupled to the touch screen sensor (10)includes touch screen driver circuitry (42) for selectively coupling thewiper layer (11) and the various contact terminals (UL,UR,LR,LL) tofirst (V_(DD)) and second (GND) reference voltages, respectively, togenerate first (V_(X)) and second (V_(Y)) analog touch location voltagesand an analog touch voltage (V_(Z)) on the resistive layer (16) at theresistive contact area (30). Analog to digital conversion circuitry (48)has an input (56) coupled to the touch screen driver circuitry (42).Multiplexing circuitry (44) in the controller (41) selectively couplesthe wiper layer (11) and various contact terminals (UL,UR,LR,LL) to theinput (56) of the analog to digital conversion circuitry (48) so ascause it to convert the first (V_(X)) and second (V_(Y)) analog touchlocation voltages and the analog touch voltage (V_(Z)) to digitalrepresentations (60) thereof, respectively.

In one embodiment, the first (UL), second (UR), third (LR), and fourth(LL) contact terminals are corner contact terminals. The touch screendriver circuitry (42) couples the second (UR) and third (LR) contactterminals to the first reference voltage (V_(DD)), the first (UL) andfourth (LL) contact terminals to the second reference voltage (GND), andthe wiper layer (11) to the input (56) of the analog to digitalconversion circuitry (48) so as to produce an analog x-coordinatevoltage (V_(X)) on the input (56) of the analog to digital conversioncircuitry (48). The touch screen driver circuitry (42) couples the first(UL) and second (UR) contact terminals to the first reference voltage(V_(DD)), the third (LR) and fourth (LL) contact terminals to the secondreference voltage (GND), and the wiper layer (11) to the input (56) ofthe analog to digital conversion circuitry (48) to produce an analogy-coordinate voltage (V_(Y)) on the input (56) of the analog to digitalconversion circuitry (48). The touch screen driver circuitry (42)couples the wiper layer (11) to the first reference voltage (V_(DD)),the third (LR) and fourth (LL) contact terminals to the second referencevoltage (GND), and the first (UL) and second (UR) contact terminals tothe input (56) of the analog to digital conversion circuitry (48) toproduce the analog touch voltage (V_(Z)) as an analog z-coordinatevoltage (V_(Z)) on the input (56) of the analog to digital conversioncircuitry (48).

In a described embodiment, a digital output of the controller (41) iscoupled by means of at least a digital bus (64) to a host processor(66), wherein the host processor (66) computes a value of the touchresistance (R_(Z)) which corresponds to the analog y-coordinate voltage(V_(Y)), the analog z-coordinate voltage (V_(Z)), and a predeterminedvalue of a touch screen resistance (R_(Y)). The host processor (66)computes a value of the touch intensity (P_(touch)) from the value ofthe touch resistance (R_(Z)) based on a predetermined relationshipbetween the touch resistance (R_(Z)) and the touch intensity(P_(touch)).

In a described embodiment, the analog to digital conversion circuitry(48) converts the analog x-coordinate voltage (V_(X)) to a digitalx-coordinate location number representative of an x-coordinate of theresistive contact area (30). The analog to digital conversion circuitry(48) also converts the analog y-coordinate voltage (V_(Y)) to a digitaly-coordinate location number representative of a y-coordinate of theresistive contact area (30). The analog to digital conversion circuitry(48) also converts the analog z-coordinate voltage (V_(Z)) to az-coordinate location number representative of the touch resistance(R_(Z)) on the contact area (30).

In a described embodiment, the host processor (66) converts the digitalx-coordinate location number to a digital x-coordinate voltage value(V_(X)) and converts the digital y-coordinate location number to adigital y-coordinate voltage value (V_(Y)). The host processor (66)converts the digital z-coordinate location number to a digitalz-coordinate voltage value (V_(Z)) and also converts the digitalz-coordinate voltage value (V_(Z)) to a digital value of the touchresistance (R_(Z)). The host processor (66) computes a value of thetouch intensity (P_(touch)) based on the digital value of the touchresistance (R_(Z)).

In a described embodiment, the touch screen driver circuitry (42)includes first (Q1), second (Q2), third (Q3) and fourth (Q5) P-channelswitching transistors having sources coupled to the first referencevoltage (V_(DD)) and drains coupled to the wiper layer (11), the secondcontact terminal (UR), the third contact terminal (LR), and the firstcontact terminal (UL), respectively. Fifth (Q4) and sixth (Q6) N-channelswitching transistors have sources coupled to the second referencevoltage (GND) and drains coupled to the third contact terminal (LR) andthe fourth contact terminal (LL), respectively. The gates of the first,second, third, fourth, fifth, and sixth switching transistors arecoupled to a touch screen driver control circuit (68) for controllingoperation of the touch screen driver circuitry (42) to measure theanalog x-coordinate voltage (V_(X)), the analog y-coordinate voltage(V_(Y)), and the analog z-coordinate voltage (V_(Z)). In a describedembodiment, a pre-processing circuit (50) is coupled between an output(60) of the analog to digital conversion circuitry (48) and the digitalbus (64) to perform filtering of digital signals on the output (60) ofthe analog to digital conversion circuitry (48).

In one embodiment, the invention provides a method for operating a5-wire touch screen system (40), including providing a 5-wire touchscreen sensor (10) that includes a wiper layer (11) and a resistivelayer (16) aligned with the wiper layer (11) and also includes first(UL), second (UR), third (LR), and fourth (LL) contact terminals,wherein a touch on the wiper layer (11) presses a small portion (31) ofthe wiper layer (11) against the resistive layer (16) thereby causing orsubstantially changing a touch resistance (R_(Z)) between a contact area(31) of the wiper layer (11) and a resistive contact area (30) of theresistive layer (16), the touch resistance (R_(Z)) being inverselyproportional to an intensity (P_(touch)) of the touch; selectivelycoupling the wiper layer (11) and various contact terminals(UL,UR,LR,LL) to first (V_(DD)) and second (GND) reference voltages,respectively, to generate an analog touch voltage (V_(Z)) on theresistive layer (16) at the resistive contact area (30), the analogtouch voltage (V_(Z)) being a function of the touch resistance (R_(Z));and selectively coupling the wiper layer (11) and various contactterminals (UL,UR,LR,LL) to an input (56) of analog to digital conversioncircuitry (48) and converting the analog touch voltage (V_(Z)) to adigital representation (60) thereof by means of the analog to digitalconversion circuitry (48).

In one embodiment, the method includes coupling the first (UL) andsecond (UR) contact terminals to the first reference voltage (V_(DD)),coupling the fourth (LL) and third (LR) contact terminals to the secondreference voltage (GND), and coupling the wiper layer (11) to the input(56) of the analog to digital conversion circuitry (48) to produce ananalog y-coordinate voltage (V_(Y)) on the input (56) of the analog todigital conversion circuitry (48); coupling the second (UR) and third(LR) contact terminals to the first reference voltage (V_(DD)), couplingthe first (UL) and fourth (LL) contact terminals to the second referencevoltage (GND), and coupling the wiper layer (11) to the input (56) ofthe analog to digital conversion circuitry (48) to produce an analogx-coordinate voltage (V_(X)) on the input (56) of the analog to digitalconversion circuitry (48); and coupling the third (LR) and fourth (LL)contact terminals to the second reference voltage (GND), coupling thewiper layer (11) to the first reference voltage (V_(DD)); and couplingthe first (UL) and second (UR) contact terminals to the input (56) ofthe analog to digital conversion circuitry (48) to produce the analogtouch voltage (V_(Z)) as an analog z-coordinate voltage (V_(Z)) on theinput (56) of the analog to digital conversion circuitry (48).

In one embodiment, the method includes coupling an output (60) of theanalog to digital conversion circuitry (48) by means of at least adigital bus (64) to a host processor (66), and operating the hostprocessor (66) to compute a value of the touch resistance (R_(Z)) whichcorresponds to the analog y-coordinate voltage (V_(X)), the analogz-coordinate (V_(Z)), and a predetermined value of a touch screenresistance (R_(Y)).

In one embodiment, the method includes operating the analog to digitalconversion circuitry (48) to convert the analog x-coordinate voltage(V_(X)) to a digital x-coordinate location number representative of anx-coordinate of the resistive contact area (30), to convert the analogy-coordinate voltage (V_(Y)) to a digital y-coordinate location numberrepresentative of a y-coordinate of the resistive contact area (30), andto convert the analog z-coordinate voltage (V_(Z)) to a digitaly-coordinate location number representative of a z-coordinate of theresistive contact area (30), wherein the host processor (66) computesthe value of the touch resistance (R_(Z)) on the basis of thez-coordinate location numbers. In one embodiment, the host processor(66) computes a value of the touch intensity (P_(touch)) from the valueof the touch resistance (R_(Z)) based on a predetermined relationshipbetween the touch resistance (R_(Z)) and the touch intensity(P_(touch)).

In one embodiment, the invention provides a 5-wire touch screen system(40) including a 5-wire touch screen sensor (10) that includes a wiperlayer (11) and a resistive layer (16) aligned with the wiper layer (11)and includes first (UL), second (UR), third (LR), and fourth (LL)contact terminals, wherein a touch on the wiper layer (11) presses asmall portion of the wiper layer (11) against the resistive layer (16)to produce a touch resistance (R_(Z)) between a contact area (31) of thewiper layer (11) and a resistive contact area (30) of the resistivelayer (16), the touch resistance (R_(Z)) being inversely proportional toan intensity (P_(touch)) of the touch; means (42) for selectivelycoupling the wiper layer (11) and various contact terminals(UL,UR,LR,LL) to first (V_(DD)) and second (GND) reference voltages,respectively, to generate an analog touch voltage (V_(Z)) on theresistive layer (16) at the resistive contact area (30), the analogtouch voltage (V_(Z)) being a function of the touch resistance (R_(Z));and means (44) for selectively coupling the wiper layer (11) to variouscontact terminals (UL,UR,LR,LL) to an input (56) of analog to digitalconversion means (48) for converting the analog touch voltage (V_(Z)) toa digital representation (60) thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded isometric diagram of a conventional 5-wireresistive touch screen.

FIG. 2 is a section view of a conventional 5-wire resistive touch screenof FIG. 1.

FIG. 3 is a diagram illustrating an equivalent circuit of a conventional5-wire resistive touch screen as shown in FIGS. 1 and 2.

FIG. 4 is a diagram illustrating an equivalent circuit of a conventional5-wire resistive touch screen as shown in FIGS. 1 and 2, where the touchpoint contact resistance R_(Z) is displayed.

FIG. 5 is a diagram illustrating an equivalent of a conventional 5-wireresistive touch screen as shown in FIG. 4, wherein wiper resistance isindicated.

FIG. 6A is a diagram of an equivalent circuit useful in explaining inthe measurement of the y-coordinate of a touch for the conventional5-wire resistive touch screens depicted in FIGS. 1-5.

FIG. 6B is a diagram of an equivalent circuit useful in explaining themeasurement of the x-coordinate of a touch for the conventional 5-wireresistive touch screens depicted in FIGS. 1-5.

FIG. 7A is a diagram of an equivalent circuit, in which the wiperresistance is assumed to be zero that is useful in explaining themeasurement of a z-coordinate representative of touch pressure appliedto the touch point of the idealized 5-wire resistive touch screensdepicted in FIGS. 1-4 as a function of y-coordinate parameters.

FIG. 7B is a diagram of an equivalent circuit, in which the wiperresistance is assumed to be zero, as in FIG. 4, that is useful inexplaining the measurement of a z-coordinate representative of touchpressure applied to the touch point of the ideal 5-wire resistive touchscreens depicted in FIGS. 1-4, as a function of x-coordinate parameters.

FIG. 8A is a diagram of a more simplified equivalent circuitrepresentation of the circuit shown in FIG. 7A.

FIG. 8B is a diagram of a more simplified equivalent circuitrepresentation of the circuit shown in FIG. 7B.

FIG. 9 is a diagram of an equivalent circuit as in FIG. 8A that furtherincludes the effects of wiper resistance as in FIG. 5 and is useful inexplaining the measurement of a z-coordinate representative of touchpressure applied to the touch point of the conventional 5-wire resistivetouch screens depicted in FIGS. 1-5.

FIG. 10 is a block diagram of a touch screen system in which the touchpressure contact area resistance measuring method and touch pressuremeasuring method of the present invention are implemented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 7A and 7B show an equivalent circuit of a touch screen 10 (FIG. 1)on which a touch pressure has been applied on a small area 31 of wiperlayer 11, thereby pressing it against the surface of top layer 12 tothereby form a resistive touch pressure contact area 30 on resistivelayer 16. Touch pressure contact areas 30 and 31 result in a contactresistance R_(Z) (or a very substantial change in the contact resistanceR_(Z)) between wiper 11 and resistive layer 16. Dashed line 33 surroundsthe touch pressure contact area resistance R_(Z) as diagrammaticallyillustrated in FIGS. 7A and 7B. Resistive layer 16 (also see FIG. 2) isrepresented as a rectangular grid of discrete resistors with terminalsUL, UR, LL, and LR in its upper left, upper right, lower left, and lowerright corners corresponding to conductive pads 15-1, 15-2, 15-4, and15-3, respectively, as shown in the exploded view in prior Art FIG. 1.

Pressure contact area resistance R_(Z) is connected in series betweenresistive layer 16 and wiper 11. The (+) terminal of a reference voltagesource 38 produces a voltage V_(DD) between the contact terminal 35 ofwiper 11 and conductor 28 as shown in FIG. 7A or between the contactterminal 35 of wiper 11 and conductor 27 as shown in FIG. 7B. In thiscase, the wiper resistance (R_(Wiper) in FIGS. 5 and 9) is assumed to bezero.

FIG. 7A shows that a resistance R_(Y) of resistive layer 16 betweenconductors 26 and 28 is equal to the sum of R_(Y1) and R_(Y2), whereR_(Y1) is the resistance in resistive layer 16 between conductor 26 andtouch pressure contact area 30 and R_(Y2) is the resistance betweentouch pressure contact area 30 and conductor 28.

The simplified equivalent circuit of FIG. 8A illustrates more clearlythan FIG. 7A the coupling of conductor 26 through resistance R_(Y1) totouch pressure contact area 30. Touch pressure contact area 30 iscoupled by the resistance R_(Y2) to conductor 28. The resistance R_(Z)between contact areas 30 and 31 (which is surrounded by dashed line 33in FIGS. 7A and 7B) is the contact resistance between resistive layer 16and wiper layer 11. To measure the touch pressure contact resistanceR_(Z) of 5-wire resistive touch screen 10 (see FIGS. 1 and 2), V_(DD) isapplied between contact terminal 35 of wiper 11 and conductor 28 (seeFIGS. 7A and 8A). The touch pressure voltage V_(Z-y) at the location oftouch pressure contact area 30 against resistive layer 16 is the voltageacross resistance R_(Y2). The voltage on conductor 26 is equal toV_(Z-Y) because the current through resistance R_(Y1) is zero, becauseconductor 26 is electrically “floating”.

Consequently, touch resistance R_(Z) can be determined by measuring thevalue of touch pressure voltage V_(Z-Y) measured between conductors 26and 28. (Note that by definition, pressure is equal to force per unitarea, and that the description of the invention herein is applicableirrespective of whether the intensity of the touch is expressed as aforce or as a pressure.) Note that V_(Z-y) is the voltage produced bythe voltage divider composed of the resistances R_(Z) and R_(Y2), andcan be represented by Equation 3:

$\begin{matrix}{V_{Z - Y} = {\lbrack \frac{R_{Y\; 2}}{( {R_{Z} + R_{Y\; 2}} )} \rbrack \times {V_{DD}.}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

To solve for the touch pressure contact R_(Z), Equation 3 can berewritten as Equation 4:

$\begin{matrix}{R_{Z} = {\lbrack \frac{( {V_{DD} - V_{Z - Y}} )}{V_{Z - Y}} \rbrack \times {R_{Y\; 2}.}}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

Replacing R_(Y2) in Equation 4 with Equation 2 results in Equation 5A:

$\begin{matrix}{R_{Z} = {( \frac{V_{Y}}{V_{DD}} ) \times ( {\frac{V_{DD}}{V_{Z - Y}} - 1} ) \times {R_{Y}.}}} & {{{Eq}.\mspace{14mu} 5}A}\end{matrix}$

Thus, the present touch resistance R_(Z) is a function of the previouslyknown values of V_(DD) and R_(Y), and the presently measured values ofV_(Z-Y) and V_(Y).

Similarly, to measure touch pressure contact resistance R_(Z) of the5-wire resistive touch screen 10 (see FIGS. 1 and 2), the voltage V_(DD)is applied between the contact terminal 35 of wiper 11 and conductor 27(see FIGS. 7B and 8B). The simplified equivalent circuit of FIG. 8Billustrates more clearly than FIG. 7B the coupling of conductor 29through resistance R_(X1) to touch pressure contact area 30. The touchpressure voltage V_(Z-X) at the location of touch pressure contact area30 against resistive layer 16 is the voltage across resistance R_(X2).The voltage produced by the voltage divider composed of the resistancesR_(Z) and R_(X2) and, with equations similar to Equation 4 and Equation5, touch pressure contact resistance to V_(Z-X) can also be expressed inEquation 5B:

$\begin{matrix}{R_{Z} = {( \frac{V_{X}}{V_{DD}} ) \times ( {\frac{V_{DD}}{V_{Z - X}} - 1} ) \times {R_{X}.}}} & {{{Eq}.\mspace{14mu} 5}B}\end{matrix}$

To measure R_(z), users can apply Equation 5A or 5B, or average theresults from both of Equations 5A and 5B. To simplify furtherdiscussion, only Equation 5A will be used.

FIG. 9 is a simplified equivalent circuit that is the same as the oneshown in FIG. 8A except that FIG. 9 further includes the resistanceR_(Wiper) of wiper 11, where R_(Wiper) includes all resistances of wiperlayer 11, including any other equivalent connection and/or wiringresistances that are coupled between touch pressure contact area 31 andthe (+) terminal of voltage source 38. In many cases, the resistanceR_(Wiper) between R_(Z) and the (+) terminal of voltage source 38 can bequite significant, due to the resistance of the resistive ITO layer ofwhich wiper layer 11 is composed (see FIG. 4) and any connection/wiringresistances between the contact terminal 35 of wiper 11 and the (+)terminal of voltage source 38. When the total resistance R_(Wiper)associated with wiper 11 is considered, Equation 5A becomes Equation 6:

$\begin{matrix}{{R_{Z} + R_{Wiper}} = {( \frac{V_{Y}}{V_{DD}} ) \times ( {\frac{V_{DD}}{V_{Z - Y}} - 1} ) \times {R_{Y}.}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

The touch resistance R_(Z) between the top wiper layer 11 and bottomresistive layer 16 is a function of the touch intensity (e.g., touchpressure or touch force), and therefore can be used to compute the touchintensity P_(touch). The touch intensity applied against any location onthe surface of a 5-wire resistive touch screen is inversely proportionalto the touch intensity contact resistance R_(Z), so a heavier touchreduces R_(Z) and a lighter touch increases R_(Z) under the exact sameconditions that determine the relationship between R_(Z) and P_(touch).As a general matter, the touch intensity P_(touch) on touch screen 10 isa function of R_(Z), and can be expressed in the polynomial form:

P _(touch) =a0+a1×R _(Z) +a2×R _(Z) ² +a3×R _(Z) ³+ . . . ,  Eq. 7

where the coefficients a0, a1, a2, a3, and so on are real values. Thecoefficients in Equation 7 are different for different touch screens.The resolution or accuracy of measuring the touch pressure contactresistance R_(Z) resulting from touching a state-of-the-art touch screenis usually quite low, and there is usually no need to use more thanabout 4 or 5 terms of Equation 7 to be able to calculate an acceptablyaccurate value of touch intensity P_(touch). The functional relationshipbetween touch resistance R_(Z) for any particular touch screen can bedetermined by a suitable calibration procedure. As a simplified example,Equation 7 may be approximated by the expression

$\begin{matrix}{P_{touch} = \frac{1}{\alpha + {\beta \times R_{Z}}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$

where the coefficients α and β are positive real values, are determinedby the touch panel structure and materials, and can be easily obtainedby the user by means of a calibration to determine the relationshipbetween P_(touch) and R_(Z).

Because the total resistance R_(Wiper) associated with wiper is aconstant at any single touch point, the touch intensity can be derivedfrom Equation 8 by substituting R_(Z) from Equation 6 and expressed byEquation 9:

$\begin{matrix}\begin{matrix}{P_{touch} = \frac{1}{\alpha + {\beta \times \lbrack {{( \frac{V_{y}}{V_{DD}} ) \times ( {\frac{V_{DD}}{V_{Z - Y}} - 1} ) \times R_{Y}} - R_{Wiper}} \rbrack}}} \\{= \frac{1}{\alpha + {\beta \times R_{Wiper}} + {\beta \times R_{Y} \times \frac{V_{Y}}{V_{DD}} \times ( {\frac{V_{DD}}{V_{Z - Y}} - 1} )}}} \\{= \frac{1}{\alpha^{\prime} + {\beta \times R_{Y} \times \frac{V_{Y}}{V_{DD}} \times ( {\frac{V_{DD}}{V_{Z.{- Y}}} - 1} )}}} \\{{= \frac{1}{a^{\prime} + {\beta \times R_{Z}}}},}\end{matrix} & {{Eq}.\mspace{14mu} 9}\end{matrix}$

where α′=α−β×R_(Wiper) is a constant at any fixed point on a 5-wireresistive touch screen. Utilizing this methodology provided by thecircuit in FIG. 9, the touch resistance at any location on a 5-wireresistive touch screen can be measured in terms of R_(Z). R_(Z) can beobtained from Equation 5A and/or 5B, or Equation 6 when considering thescreen resistance of wiper layer 11.

Comparing Equation 9 (where R_(Wiper) is considered, as shown in FIG. 9)with Equation 8 (where R_(Wiper) is not considered, as in FIGS. 8A and8B), the expression of the relationship between R_(Z) and P_(touch) isthe same at every touch area on a 5-wire restive touch screen.

FIG. 10 shows a touch screen system 40 which includes touch screen 10coupled to a touch screen controller 41 that can interface with a hostprocessor 66. Touch screen system 40 provides digital representations ofmeasured values of the V_(X), V_(Y), and V_(Z) (i.e., V_(Z-Y) orV_(Z-X)) voltages expressed in the foregoing equations. Or alternativelyand often preferably, touch screen system 40 can provide digital x, y,and z “coordinate values” representative of the measured values of theV_(X), V_(Y), and V_(Z) voltages, which completely indicate athree-dimensional touch location on touch screen 10.

There are 5 analog signals coupled between touch screen controller 41and touch screen 10. Touch screen controller 41 is connected to contactterminal 35 of wiper 11 of touch screen 10. Touch screen controller 41also is connected to terminals UL, UR, LR, and LL of touch screen 10.Terminals UL, UR, LR, and LL and contact terminal 35 of wiper 11 areconnected to touch screen driver circuitry 42 inside touch screencontroller 41, and are further connected to the inputs of a multiplexer44 of touch screen controller 41. Multiplexer 44 determines which ofthese conductors are multiplexed to the input 56 of an ADC (analog todigital converter) 48 which converts V_(X), V_(Y), and V_(Z) to digitaltouch data. After preprocessing circuit 50 (which, for example, canperform noise filtering), digital touch data is sent to host processor66 through a conventional digital interface control circuit 54 and adigital bus 64.

Wiper 11 is connected by contact terminal 35 to the drain of a P-channelswitching transistor Q1 having its source connected to V_(DD). V_(DD)also is connected to the sources of P-channel switching transistors Q2,Q3, and Q5. The drains of transistors Q2, Q3 and Q5 are connected to URterminal 15-2, LR terminal 15-3, and UL terminal 15-1, respectively. Thesources of N-channel switching transistors Q4 and Q6 are connected toground. The drain of transistor Q4 is connected to LR terminal 15-3, andthe drain of transistor Q6 is connected to UL terminal 15-1. The gatesof transistors Q1, 2 . . . 6 are connected to a driver controllercircuit 68 of touch screen driver 42, which can be controlled accordingto either simple logic circuitry or according to appropriate controlsignals or commands from host processor 66.

Touch screen system 40 can be considered to include touch screen 10,touch screen controller 41, and a portion of host processor 66. Aportion 66A of host processor 66 which can be considered to be part oftouch screen system 40 is the portion that communicates with touchscreen controller 41 through digital interface control circuit 54 andtouch detector 46. Portion 66A can be considered to include softwarethat performs the above described calculations associated with touchscreen controller 41 and software that is associated with operation oftouch screen driver 42. Portion 66A of host processor 66 also can beconsidered to include software and hardware that is associated withstoring data associated with touch screen 10 and communicating the datato application software elsewhere in host processor 66.

The switch transistors in driver controller 68 can be easily controlledby various circuitry, such as a simple state machine, that implementssubsequently described Table 1. Driver controller 68 can receive acommand from host processor 66 via conductor or bus 65 and digitalinterface control circuit 54.

Multiplexer 44 multiplexes the 5 signals on conductors 35, 15-1, 15-2,15-3 and 15-4 from touch screen 10 to generate reference voltagesV_(REF) ⁺ and V_(REF) ⁻ and also generate an analog input signal oninput conductor 56 of ADC 48. (PENIRQ is an interrupt output from atouch detector circuit 46 having an input connected to wiper contactterminal 35, and indicates if a touch on touch screen 10 has beendetected.)

Table 1 shows the states of the various transistors (or switches) Q1-6and the connections of the various terminals of resistive layer 16 andwiper 11 (i.e., the analog inputs to multiplexer 44) and the voltagereference signals and the analog signal to ADC 48 that are output frommultiplexer 44 during operation of touch screen controller 40 to measureV_(X), V_(Y), and V_(Z).

TABLE 1 Measuring V_(X) Measuring V_(Y) Measuring V_(Z) ON OFF ON OFF ONOFF Input to Multiplexer 44 Wiper: --> Q1 Q1 Q1 UR: --> Q2 Q2 Q2 LR: -->Q3 Q4 Q4 Q3 Q4 Q3 UL: --> Q6 Q5 Q5 Q6 Q5, Q6 LL: --> always connected toGND Output from Multiplexer 44 ADC Wiper contact 35 wiper contact 35UL/UR input 56: --> V_(REF) ⁺: --> UR/LR UL/UR Wiper contact 35 V_(REF)⁻: --> UL/LL LR/LL LR/LL

ADC (analog to digital converter) 48 converts the measured analogvoltages V_(X), V_(Y), and V_(Z) on conductor 56 to a digital value ondigital bus 60 in accordance with the various conditions indicated inTable 1.

As previously indicated, wiper contact 35 is selectively coupled toV_(DD) through Q1, and UR is selectively coupled to V_(DD) through Q2.LR is selectively coupled to V_(DD) through Q3 and to ground through Q4.UL is selectively coupled to V_(DD) through Q5 and to ground through Q6.

Referring to Table 1, to measure V_(X), Q1 is off, so wiper contact 35is coupled through multiplexer 44 to ADC input 56. Q2 is on, so UR iscoupled to V_(DD). Q3 is on, so LR is coupled to V_(DD). Q3 and Q4cannot both be on at the same time, and Q3 is on, so Q4 is off. Q5 isoff and Q6 is on, which means UL and LL both are coupled to ground whileUR and LR both are coupled to V_(DD). Q2 and Q3 both are on UR and LRboth are at V_(DD). Wiper contact 35 is electrically floating because Q1is off. UL is at ground because Q6 is on, and LL is always at ground.The V_(REF) ⁺ reference voltage input of ADC 48 is connected to UL andLR, which results in a voltage nearly equal to V_(DD) being coupled tothe V_(REF) ⁺ input of ADC 48. The V_(REF) ⁻ reference voltage input ofADC 48 is connected to UL and LL, which results in a voltage nearlyequal to ground being connected to the V_(REF) ⁻ reference voltage inputof ADC 48. See FIG. 6B.

To measure V_(Y), Q1 is off, so wiper contact 35 is coupled throughmultiplexer 44 to ADC input 56. Q2 is on, so UR is coupled to V_(DD). Q4is on, so LR is coupled to GND. Q3 and Q4 cannot both be on at the sametime, and Q4 is on, so Q3 is off. Q5 is on and Q6 is off, which means LRand LL both are coupled to ground while UR and UL both are coupled toV_(DD). Since Q2 and Q4 are on, UR and UL both are at V_(DD). Wipercontact 35 is electrically floating because Q1 is off. LR is at groundbecause Q5 is on, and LL is always at ground. The V_(REF) ⁺ referencevoltage input of ADC 48 is connected to UL and UR, which results in avoltage nearly equal to V_(DD) being coupled to the V_(REF) ⁺ input ofADC 48. The V_(REF) ⁻ reference voltage input of ADC 48 is connected tothe LR and LL, which results in a voltage nearly equal to ground beingconnected to the V_(REF) ⁻ reference voltage input of ADC 48. See FIG.6A.

To measure V_(Z), Q1 is on so wiper contact 35 is connected to V_(DD).Q2 is off so UR is electrically floating. Q3 is off and Q4 is on, so LRis at ground. Q5 and Q6 both are off so UL is electrically floating. LLis at ground. The input of the analog to digital conversion circuitry isconnected to UL and UR. The V_(REF) ⁺ reference voltage input of ADC 48is connected to wiper contact 35. The V_(REF) ⁻ reference voltage inputof ADC 48 is connected to LR and LL. See FIGS. 7A and 8A.

The digital output generated on digital bus 60 by ADC 48 is provided asan input to a pre-processing circuit 50, which can function as a digitalaveraging filter to reduce noise before sending the measured quantity tohost processor 66. Pre-processing circuit 50 also can perform variousother functions, such as data validation. The output of pre-processingcircuit 50 is coupled by digital bus 62 to a digital interface controlcircuit 54, which is coupled by means of bidirectional digital bus 64 tohost processor 66.

Touch screen 10 and touch driver 42 produce values of analog voltagesV_(X), V_(Y), and V_(Z) to the input 56 of ADC 48. V_(X), V_(Y), andV_(Z) represent three-dimensional touch position coordinates on touchscreen 10, namely V_(X), V_(Y), and V_(Z) as expressed by Equations 1 to3, respectively. Typically, the digitized values of V_(X), V_(Y), andV_(Z) produced by ADC 48 actually are digital x, y, and z coordinatelocation numbers, e.g. 2046, 4096 or the like corresponding to each ofthe analog values of V_(X), V_(Y), and V_(Z) produced by multiplexer 44on the input 56 of ADC 48. ADC 48 performs the conversions of the analogvalues of V_(X), V_(Y), and V_(Z) to the digital x, y, and z coordinatelocation numbers and provides them to host processor 66 viapreprocessing circuit 50 and digital interface control 54. Hostprocessor 66 presents, applies, and/or interprets the data for specificuser applications.

Thus, the digital representations of V_(X), V_(Y), and V_(Z) (i.e.,V_(Z-Y) or V_(Z-X)) for the current touch on touch screen 10, forexample, digital x, y, and z touch screen coordinate location numberrepresentations of the analog voltages V_(X), V_(Y), and V_(Z), areprovided by the controller 41 to host processor 66. If directlydigitized representations of the measured analog voltages V_(X), V_(Y),and V_(Z) are provided by driver 42, then host processor 66 then can usethat information to locate the touch position corresponding to V_(X) andV_(Y), compute values of R_(Z), and further compute the value ofP_(touch). Host processor 66 then can use those values for the presentuser application or purpose.

Host processor 66 might then use the value of R_(Z) to eliminate systemnoise and/or improve the accuracy of the touch point information inother ways. For example, the z coordinate information may help determinewhether what appears to be a very light pressure touch point is actuallyjust due to vibration.

The relationship between touch point resistance R_(Z) and touch pressureor intensity P_(touch) may be complex, and various users may use hostprocessor 66 to execute various algorithms to compute the touch pressureor intensity P_(touch) on the basis of the digital representations ofV_(X), V_(Y), and V_(Z) generated by touch screen system 40 shown inFIG. 10. Host processor 66 can be used to establish/calibrate therelationship between P_(touch) and R_(Z) for any particular touch screen10 and compute the touch pressure or intensity P_(touch) being appliedto wiper layer 11 on the basis of values of V_(X), V_(Y), and V_(Z)generated by touch screen system 40.

It should be appreciated that the present invention is believed toprovide the first S-wire touch screen system that generates measurementsfrom which a third dimensional coordinate value R_(Z) can be obtained atany point on a 5-wire touch screen and touch point pressure can becomputed, and thereby enables the host processor to perform morefunctions with more accuracy than previously has been possible using5-wire touch screen systems. This can be very useful in someapplications, such as graphic drawing, determining line or dot size,signature verification, in which the touch intensity applied to providea valid signature can be very significant. With theintensity/z-coordinate technique of the present invention, the touchscreen system can generate 3-dimensional coordinates and thereforesupports “3-dimensional” or “real-world” applications. In general,information regarding how much touch intensity is applied to the touchscreen can help improve the overall performance of the touch screensystem.

While the invention has been described with reference to severalparticular embodiments thereof, those skilled in the art will be able tomake various modifications to the described embodiments of the inventionwithout departing from its true spirit and scope. It is intended thatall elements or steps which are insubstantially different from thoserecited in the claims but perform substantially the same functions,respectively, in substantially the same way to achieve the same resultas what is claimed are within the scope of the invention. For example,in addition to using the above mentioned state machine to implementdriver controller 68, there are other ways of controlling touch screendriver 42. Driver controller 68 could be implemented by means of logiccircuitry other than a state machine. Host processor 66 may initiateoperation of driver controller 68 so as to cause operation of touchscreen driver 42 in accordance with Table 1. Driver controller 68 itselfcould be programmable so as to cause touch screen driver 42 toautomatically operate as desired to measure V_(X), V_(Y) and V_(Z) if atouching on the touch screen surface is detected. Alternatively, thepreprocessing circuitry 50 could be configured to control drivercontroller 68 in response to a valid touch on the surface of touchscreen 10.

1. A 5-wire touch screen system comprising: a 5-wire touch screen sensorincluding a substantially transparent, substantially conductive wiperlayer, a substantially transparent resistive layer aligned with thewiper layer, the resistive layer including first, second, third, andfourth contact terminals, and a plurality of thin spacers separating thewiper layer and the resistive layer, wherein a touch on the wiper layerpresses a small portion of the wiper layer against the resistive layerto form a resistive contact area having a touch resistance between thewiper layer and the resistive layer, the touch resistance beinginversely proportional to an intensity of the touch; and a controllercoupled to the touch screen sensor, including touch screen drivercircuitry for selectively coupling the wiper layer and the variouscontact terminals to first and second reference voltages, respectively,to generate first and second analog touch location voltages and ananalog touch voltage on the resistive layer at the resistive contactarea, analog to digital conversion circuitry having an input coupled tothe touch screen driver circuitry, and multiplexing circuitry in thecontroller for selectively coupling the wiper layer and the variouscontact terminals to the input of the analog to digital conversioncircuitry so as cause it to convert the first and second analog touchlocation voltages and the analog touch voltage to digitalrepresentations thereof, respectively.
 2. The 5-wire touch screen systemof claim 1 wherein: the first, second, third, and fourth contactterminals are corner contact terminals, the touch screen drivercircuitry couples the second and third contact terminals to the firstreference voltage, the first and fourth contact terminals to the secondreference voltage, and the wiper layer to the input of the analog todigital conversion circuitry to produce an analog x-coordinate voltageon the input of the analog to digital conversion circuitry, the touchscreen driver circuitry couples the first and second contact terminalsto the first reference voltage, the third and fourth contact terminalsto the second reference voltage, and the wiper layer to the input of theanalog to digital conversion circuitry to produce an analog y-coordinatevoltage on the input of the analog to digital conversion circuitry, andthe touch screen driver circuitry couples the wiper layer to the firstreference voltage, the third and fourth contact terminals to the secondreference voltage, and the first and second contact terminals to theinput of the analog to digital conversion circuitry to produce theanalog touch voltage as an analog z-coordinate voltage on the input ofthe analog to digital conversion circuitry.
 3. The 5-wire touch screensystem of claim 2 wherein a digital output of the controller is coupledby means of at least a digital bus to a host processor, wherein the hostprocessor computes a value of the touch resistance which corresponds tothe analog y-coordinate voltage, the analog z-coordinate voltage, and apredetermined value of a touch screen resistance.
 4. The 5-wire touchscreen system of 2 wherein the host processor computes a value of thetouch intensity from the value of the touch resistance based on apredetermined relationship between the touch resistance and the touchintensity.
 5. The 5-wire touch screen system of claim 2 wherein theanalog to digital conversion circuitry converts the analog x-coordinatevoltage to a digital x-coordinate location number representative of anx-coordinate of the resistive contact area, wherein the analog todigital conversion circuitry also converts the analog y-coordinatevoltage to a digital y-coordinate location number representative of ay-coordinate of the resistive contact area, and the analog to digitalconversion circuitry also converts the analog z-coordinate voltage to az-coordinate location number representative of the touch resistance onthe contact area.
 6. The 5-wire touch screen system of claim 5 whereinthe host processor converts the digital x-coordinate location number toa digital x-coordinate voltage value and converts the digitaly-coordinate location number to a digital y-coordinate voltage value. 7.The 5-wire touch screen system of claim 6 wherein the host processorconverts the digital z-coordinate location number to a digitalz-coordinate voltage value and also converts the digital z-coordinatevoltage value to a digital value of the touch resistance.
 8. The 5-wiretouch screen system of claim 7 wherein the host processor computes avalue of the touch intensity based on the digital value of the touchresistance.
 9. The 5-wire touch screen system of claim 8 wherein thevalue of the touch intensity is computed according to the equation$P_{touch} = \frac{1}{\alpha^{\prime} + {\beta \times R_{Z}}}$ whereα′=α−β×R_(Wiper) is a constant at any fixed point on the 5-wire touchscreen sensor and β is a positive real value determined by structure andmaterials of the 5-wire touch screen sensor.
 10. The 5-wire touch screensystem of claim 2 wherein the touch screen driver circuitry includes:first, second, third and fourth P-channel switching transistors havingsources coupled to the first reference voltage and drains coupled to thewiper layer, the second contact terminal, the third contact terminal,and the first contact terminal, respectively, fifth and sixth N-channelswitching transistors having sources coupled to the second referencevoltage and drains coupled to the third contact terminal and the fourthcontact terminal, respectively, and gates of the first, second, third,fourth, fifth, and sixth switching transistors being coupled to a touchscreen driver control circuit for controlling operation of the touchscreen driver circuitry to measure the analog x-coordinate voltage, theanalog y-coordinate voltage, and the analog z-coordinate voltage. 11.The 5-wire touch screen system of claim 10 wherein the touch screendriver control circuit includes a state machine.
 12. The 5-wire touchscreen system of claim 1 including a touch detector circuit forgenerating an interrupt signal in response to a touch detection signalreceived from the wiper layer.
 13. The 5-wire touch screen system ofclaim 3 including a pre-processing circuit coupled between an output ofthe analog to digital conversion circuitry and the digital bus forperforming the function of filtering digital signals on the output ofthe analog to digital conversion circuitry.
 14. The 5-wire touch screensystem of claim 1 wherein the spacers are elastic.
 15. A method foroperating a 5-wire touch screen system, comprising: providing a 5-wiretouch screen sensor including a wiper layer and a resistive layeraligned with the wiper layer and including first, second, third, andfourth contact terminals, wherein a touch on the wiper layer presses asmall portion of the wiper layer against the resistive layer to change atouch resistance between a contact area of the wiper layer and aresistive contact area of the resistive layer, the touch resistancebeing inversely proportional to an intensity of the touch; selectivelycoupling the wiper layer and various contact terminals to first andsecond reference voltages, respectively, to generate an analog touchvoltage on the resistive layer at the resistive contact area, the analogtouch voltage being a function of the touch resistance; and selectivelycoupling the wiper layer and the various contact terminals to an inputof analog to digital conversion circuitry and converting the analogtouch voltage to a digital representation thereof by means of the analogto digital conversion circuitry.
 16. The method of claim 15 includingcoupling the first and second contact terminals to the first referencevoltage, coupling the fourth and third contact terminals to the secondreference voltage, and coupling the wiper layer to the input of theanalog to digital conversion circuitry to produce an analog y-coordinatevoltage on the input of the analog to digital conversion circuitry, themethod also including coupling the second and third contact terminals tothe first reference voltage, coupling the first and fourth contactterminals to the second reference voltage, and coupling the wiper layerto the input of the analog to digital conversion circuitry to produce ananalog x-coordinate voltage on the input of the analog to digitalconversion circuitry, the method also including coupling the third andfourth contact terminals to the second reference voltage coupling thewiper layer to the first reference voltage, and coupling the first andsecond contact terminals to the input of the analog to digitalconversion circuitry to produce the analog touch voltage as an analogz-coordinate voltage on the input of the analog to digital conversioncircuitry.
 17. The method of claim 16 including coupling an output ofthe analog to digital conversion circuitry by means of at least adigital bus to a host processor, and operating the host processor tocompute a value of the touch resistance which corresponds to the analogy-coordinate voltage, the analog z-coordinate, and a predetermined valueof a touch screen resistance.
 18. The method of claim 17 includingoperating the analog to digital conversion circuitry to convert theanalog x-coordinate voltage to a digital x-coordinate location numberrepresentative of an x-coordinate of the resistive contact area, toconvert the analog y-coordinate voltage to a digital y-coordinatelocation number representative of a y-coordinate of the resistivecontact area, and to convert the analog z-coordinate voltage to adigital y-coordinate location number representative of a z-coordinate ofthe resistive contact area, and wherein the host processor computes thevalue of the touch resistance on the basis of the z-coordinate locationnumbers.
 19. The method of claim 18 wherein the host processor computesa value of the touch intensity from the value of the touch resistancebased on a predetermined relationship between the touch resistance andthe touch intensity.
 20. A 5-wire touch screen system comprising: a5-wire touch screen sensor including a wiper layer and a resistive layeraligned with the wiper layer and including first, second, third, andfourth contact terminals, wherein a touch on the wiper layer presses asmall portion of the wiper layer against the resistive layer to change atouch resistance between a contact area of the wiper layer and aresistive contact area of the resistive layer, the touch resistancebeing inversely proportional to an intensity of the touch; means forselectively coupling the wiper layer and various contact terminals tofirst and second reference voltages, respectively, to generate an analogtouch voltage on the resistive layer at the resistive contact area, theanalog touch voltage being a function of the touch resistance; and meansfor selectively coupling the wiper layer to various contact terminals toan input of analog to digital conversion means for converting the analogtouch voltage to a digital representation thereof.